Conventionally, a known structure of such an engine oil supply apparatus comprises a first oil pump for pumping up hydraulic oil from an oil pan and a second oil pump for pumping up hydraulic oil from an oil bath. In the known structure, an oil feeding path communicating a hydraulic oil consumption section with the first oil pump is connected therebetween, a valve system lubrication branching oil passage communicating with each lubrication section of a valve system is branched from the oil feeding path. The oil bath is capable of receiving any oil that has leaked from the lubrication sections and allows any excess oil to overflow to the oil pan (See, for example, Patent Document 1: Japanese Patent No. 3023803 (paragraph number 0033)).
In addition, the second oil pump for pumping up hydraulic oil from the oil bath is connected to a hydraulic chamber of valve operation characteristic changing means.
Accordingly, it is possible to reduce an increase in friction loss during high-speed rotation of an engine by setting the volume of the second oil pump so as to have no relationship with the amount of oil required at the lubrication sections. In addition, maintenance can be simplified by connecting the oil bath and the oil pan.
Another type of oil supply apparatus has also been available. This engine supplier comprises a first oil pump serving as a main pump in a control hydraulic circuit of an engine driving mechanism, a second oil pump serving as a sub-pump which is different from the first oil pump and disposed in parallel, and a switching mechanism for switching between oil passages of both of the oil pumps depending upon the driving state of the engine. (See, for example, Patent Document 2: Japanese Unexamined Utility Model Registration Application Publication No. 4-132414 (paragraph numbers 0006 to 0009).
Accordingly, for example, when the engine is rotating at a low speed and is subjected to a high load, it is possible to connect the second oil pump to the control hydraulic circuit and disconnect the first oil pump from the control hydraulic circuit by the switching mechanism. Therefore, since a sufficient hydraulic oil pressure in accordance with a required amount is quickly supplied from the second oil pump (not used for, for example, lubrication) to a phase changing mechanism used for changing the relative rotational phases of a cam shaft and a sprocket, the operation speed of the phase changing mechanism is high.
Patent Document 2 also discloses an engine oil supply apparatus comprising a second oil pump disposed in series in a control oil passage circuit of the driving mechanism in order to increase the discharging capability of the first oil pump. Accordingly, for example, when the engine is rotating at a low speed and is subjected to a high load, the hydraulic oil which is discharged from the first oil pump and which has a portion sent under pressure to the control hydraulic circuit has its discharge pressure further increased by the second oil pump. Therefore, since a sufficient hydraulic oil pressure in accordance with a required amount is quickly provided at the phase changing mechanism, the operating speed of the phase changing mechanism is high.
Ordinarily, oil at any portion of the engine requiring lubrication returns to an oil holder for holding oil, such as an oil pan, through gaps between engine members prior to starting the engine. Therefore, prior to the starting of the engine, the oil pumps and oil passages are not filled with oil, and, thus, a certain amount of air exists. Consequently, after starting the engine, it is necessary to supply oil as quickly as possible to the oil pumps and the oil passages and to supply oil to any parts of the engine requiring lubrication in order to reduce friction at sliding sections and to thus achieve smooth movement.
Accordingly, since it is necessary for the first and second oil pumps to supply oil to any portion of the engine requiring lubrication in a short time while removing air from the oil passages, it is desirable for the oil pumps to be strong. In particular, the second oil pump forcefully draws in oil from the oil holder through a slight gap in the first oil pump when, for example, the crank shaft is driven due to the starting of the engine. This causes the second oil pump to have a larger size. Ordinarily, since the second oil pump is disposed at the control hydraulic circuit of the engine driving mechanism, it is not desirable for the second oil pump to be large.
Further, in the structure of Patent Document 2 in which the first oil pump and the second oil pump which increases the discharge pressure of the first oil pump are disposed in series, the second oil pump draws in the oil drawn in by and discharged from the first oil pump. Therefore, the suction of the oil by the second oil pump requires suction and discharging of oil by the driving of the first oil pump. This results in the problem that the second oil pump cannot draw in oil unless the first oil pump is driven and discharges oil. As a result, smooth movement may not be achieved for some time after the driving of the first oil pump at, for example, the sliding sections of the engine to which oil is supplied from the second pump.
Accordingly, it is an object of the present invention to provide an engine oil supply apparatus comprising a second oil pump which can draw in and discharge oil quickly regardless of whether or not a first oil pump is driven and which can increase the discharge pressure of the first oil pump without increasing the size of the second oil pump.